The wide direct bandgap (6.2 eV), high thermal conductivity (3.2 W/cmK), and high electron drift velocity of aluminum nitride (AlN) make it an excellent candidate for high-power, high-frequency electronic and deep-UV optoelectronic devices. Group III-nitrides grown on sapphire and SiC substrates are commercially available. However, the rather severe mismatch between the aforementioned substrates and overgrown active layers limits device lifetime and performance.
The use of native substrates, such as crystalline AlN wafers, in the group III-nitride (III-N) device fabrication process will reduce the dislocation density in the overgrown films below 1000 cm−2 and drastically improve both device performance and lifetime. Bulk growth of III-N crystals is challenging due to the process thermodynamics and extreme operating conditions. AlN or gallium nitride (GaN) single crystals of sizes suitable for substrate applications are not available.
The sandwich-sublimation technique is very promising for growth of large AlN crystals. Self-seeded growth and seeded growth on AlN seeds by powder sublimation has been shown to induce minimal stresses and, hence, nearly dislocation-free crystals can be achieved. In vertical cross-sections of a polycrystalline, self-seeded AlN boule, a gradual grain expansion has been observed in the growth direction. A number of consecutive growth runs are needed to achieve a large single crystalline AlN by grain expansion starting with a polycrystalline material. However, secondary nucleation has been identified as a major issue when growing on AlN seed crystals that have been exposed to air or cut and polished. In addition, oxygen is a very common impurity in AlN crystals. It has a significant influence on the electrical, optical, and thermal properties of the material. For example, oxygen acts as a deep donor and is thought to induce broad absorption bands in the range of 3.5 to 4.5 eV. Oxygen concentration must be minimized for fabrication of high-quality, AlN-based optoelectronic and electronic devices. The effect of oxygen and other impurities on the growth of AlN by physical vapor transport (PVT) is increasingly discussed in the art.
As noted above, secondary nucleation has been a major issue in the growth of bulk AlN on single crystalline AlN seeds that have been previously cut/polished or exposed to air. Secondary nucleation may result for several reasons, including low temperature deposition during ramp-up to the growth conditions and the presence of surface oxide and surface damage caused by cutting and polishing the seed. Impurities that originate from the source and the growth atmosphere can also enhance secondary nucleation. At low temperatures, such as 1700-1800° C., in the PVT growth of bulk AlN by powder sublimation in nitrogen atmosphere, randomly oriented AlN containing Al—O—N is deposited in the form of a white polycrystalline material. It is believed that in this temperature range, oxygen-assisted transport of Al species takes place and AIO and Al2O exist in the gas phase. Oxygen atoms may originate from the powder source and/or the growth environment. Furthermore, an excess of Al in the powder source promotes an early supersaturation of Al species at a lower temperature because of the lower activation energy for breaking Al—Al bonds as compared to Al—N bonds. Supersaturation of Al can further result in a fast, low temperature deposition of Al with impurities by similar mechanism as oxygen-assisted transport through intermediate species in the vapor phase. As noted above, the presence of a surface oxide on AlN seeds would affect the ordering of adatoms and possibly result in random growth direction. These provide a number of defective positions with minimum energy that act as nucleation sites and hence promote random nucleation.
There is a need in the art for a physical vapor transport process that produces bulk single crystal AlN using an AlN seed material in which seed surface contamination is removed and the crystallinity of the seed is reproduced in the growing crystal.